1. Field of the Invention
This invention relates to a desensitizing gum for planographic printing plates.
2. Description of the Prior Art
In the final step of a process for making planographic printing plates, a protective coating composition, a so-called desensitizing gum, is applied to the plate surface for protecting the plate from contamination and scratching during storage. This step is called "a gumming-up" step. The most important purpose of gumming-up is to prevent the plate surface from being brought into contact with air and to thereby prevent the hydrophilic property in non-image areas from being reduced due to air oxidation, as well as to increase hydrophilicity. Thus, the gumming-up step is an essential step in making printing plates.
As a desensitizing gum, there has been used about 15 to 30 wt. % aqueous solution of gum arabic optionally containing surfactants, pH regulating agents, preservatives, etc.
However, gum arabic is a natural product which is produced only in restricted areas of the world, and therefore, the yield of gum arabic depends on the weather in such areas, which makes the supply thereof unstable.
For this reason, many attempts have been made to use as a desensitizing gum for printing plates various water-soluble organic high molecular compounds instead of gum arabic. For example, British Pat. No. 2,010,298 discloses dextrin, arabogalactan, arginic acid salts, polyacrylic acids, hydroxyethyl cellulose, polyvinyl pyrrolidone, polyacrylamide, methyl cellulose, hydroxypropyl cellulose, hydroxymethyl cellulose, and carboxyalkyl cellulose salts; U.S. Pat. No. 4,095,525 discloses pullulan and its derivatives, and; U.S. Pat. No. 4,349,391 discloses polyvinyl alcohol, all of which are inferior to gum arabic in ability of desensitizing non-image areas.